(1) Field of the Invention
The present invention relates to a semiconductor integrated circuit having improved load drive characteristics. More particularly, it relates to a semiconductor integrated circuit comprising an analog buffer amplifier able to output a predetermined voltage level in accordance with an input signal voltage level irrespective of a value of a load, without consuming high power. Also, it relates to a semiconductor integrated circuit comprising a constant current output circuit utilizing the above analog buffer amplifier, able to output a predetermined constant current in accordance with a constant input signal voltage level and a resistance value of a variable resistor connected from outside of a chip to a predetermined terminal of the circuit.
(2) Description of the Related Art
Generally, a conventional semiconductor integrated circuit used as an analog buffer amplifier comprises a differential stage and an output stage, and an input signal having a voltage level V.sub.in is input to a input terminal provided in the differential stage.
When the input signal level V.sub.in rises, the output voltage level of the differential stage falls, and this lower voltage level is then inverted by the output stage. As a result, an output voltage having a voltage level V.sub.out, which is nearly equal to the input signal voltage level V.sub.in, is output from the output stage irrespective of the value of a load connected to an output terminal provided in the output stage.
However, in the above-mentioned conventional analog buffer amplifier, the output voltage V.sub.out of the output stage is fed back to the differential stage, and as a result, oscillation may occur if the frequency characteristics of the load L are in a predetermined condition.
Therefore, a capacitor is provided in the output stage to correct the phase of the output voltage V.sub.out and prevent the above oscillation.
As a result, a large area in the semiconductor integrated circuit is occupied by the capacitor, and thus a problem arises in that the total area needed for the layout of the semiconductor integrated circuit tends to increase. In this connection, if the capacitor is small in size, the range of the load which can be driven by the analog buffer amplifier is limited.
Further, in the above conventional semiconductor integrated circuit, a constant current flows through constant current sources provided in the differential stage and the output stage, irrespective of the value of the load driven by the analog buffer amplifier. Particularly, in the constant current source provided in the output stage, it is necessary to determine the value of the constant current flowing through the constant current source by taking into consideration the value of the constant current flowing when the maximum load is driven.
As a result, another problem arises in that the constant current value is always equal to the value needed when the maximum load is driven, and thus power is wasted.